Light energy emitted by LiDAR system is known as Pulse……
light reflected from the ground/object is known as Return….

A LiDAR unit scans the ground from side to side as the plane flies because this covers a larger area.

While some pulses will be directly at nadir [direct line], most pulses travel at an angle (off-nadir). When a LiDAR system calculates elevation, it needs to accounts for angle.

Airplanes and helicopters are the most commonly used platforms for acquiring LiDAR data over broad areas

There are different ways to collect data using LiDAR
A. from ground
B. From plane
C. from satellite

And there are two types of LiDAR….Topographic LiDAR…. It maps the land typically using near-infrared light.
Bathymetric LiDAR…. it uses water-penetrating green light to measure seafloor and riverbed elevations.

How do a light detection and ranging system work? There are 4 parts of an airborne LiDAR.
LiDAR sensors scan the ground from side to side as the plane flies.GPS receivers track the altitude and location of the airplane. These variables are important in attaining accurate terrain elevation values.Inertial measurement units (IMU) tracks the tilt of the airplane as it flies. Elevation calculations use tilt to accurately measure the incident angle of the pulse.
Computers record all of the height information as the LiDAR scans the surface.

These 4 parts of a LiDAR system work together to produce highly accurate, usable results.